[GitHub] [arrow] westonpace commented on pull request #11014: ARROW-13775: [C++] Allow Partitioning objects to be created with a vector of field names

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westonpace commented on pull request #11014:
URL: https://github.com/apache/arrow/pull/11014#issuecomment-908595122


   > What would be a use case? (I don't see any test where it is actually used for something) I suppose as alternative for #11008, right?
   
   Yes, but not as an alternative, this would be to enable #11008.  I don't believe #11008 works today.  `ds.partitioning(field_names=['a'])` returns a partitioning factory and not a partitioning.  It also returns an error when specifying the hive format.  A partitioning factory cannot be used for writing datasets (only reading)...
   
   ```
   >>> import pyarrow
   >>> import pyarrow as pa
   >>> import pyarrow.dataset as ds
   >>> table = pa.Table.from_pydict({'a': [1, 2, 3], 'b': ['x', 'y', 'z']})
   >>> ds.partitioning(field_names=['a'])
   <pyarrow._dataset.PartitioningFactory object at 0x7f2410d4c170>
   >>> part = ds.partitioning(field_names=['a'])
   >>> ds.write_dataset(table, '/tmp/new_dataset', format='parquet', partitioning=part)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "/home/pace/anaconda3/envs/arrow-release-5/lib/python3.9/site-packages/pyarrow/dataset.py", line 791, in write_dataset
       partitioning = _ensure_write_partitioning(partitioning)
     File "/home/pace/anaconda3/envs/arrow-release-5/lib/python3.9/site-packages/pyarrow/dataset.py", line 686, in _ensure_write_partitioning
       raise ValueError("partitioning needs to be actual Partitioning object")
   ValueError: partitioning needs to be actual Partitioning object
   ```
   
   I didn't add real world use cases (or python bindings) because I figured those would be covered by #11008.


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